Soybean oil is currently the predominant plant oil in the world. However, soybean oil is relatively unstable to oxidation and therefore its use is limited to applications in which a high degree of oxidative stability is not required. Soybean oil contains high levels of polyunsaturated fatty acids and is more prone to oxidation than oils with higher levels of monounsaturated and saturated fatty acids. The higher the degree of unsaturation in an oil the more likely that the oil will go rancid (oxidize). Oxidation leads to the development of off flavors and odors in the oil as a result of the degradation process. Oils with high levels of polyunsaturated fatty acids are not often used in applications that require a high degree of oxidative stability, such as cooking for a long period of time at an elevated temperature.
Several methods are available to increase the stability of soybean oil. One commonly used method is catalytic hydrogenation, a process that reduces the number of double bonds and raises the melting point of the fat. Consequently, hydrogenation also increases the saturated fatty acid content of oil. Another approach to increase oxidative stability is through the addition of antioxidants.
Each of these approaches suffers from one or more drawbacks, for example, hydrogenation of oils has been linked to health, environmental and food quality concerns. One known consequence of hydrogenation of oils is the production of trans fatty acid isomers which have been associated with deleterious health effects including increased risk of coronary heart disease. (Food Product Design, November 1994). In the case of antioxidants, some are very expensive to acquire and not all antioxidants withstand high temperatures. In addition, in many cases a food manufacturer does not want to use oils with added antioxidants if a label with unadulterated ingredients is desired. Thus, an oil which has a high oxidative stability under high temperatures without requiring the addition of antioxidants is very desirable.
U.S. Pat. No. 5,260,077, issued to Carrick et al. on Nov. 9, 1993, discloses a method of stabilizing triglyceride oils of high oleic acid content by addition of tocopherol, a natural antioxidant. The combination of the high oleic oil and tocopherol results in a stable composition suitable for deep frying.
World Patent Publication W094/11516, published on May 26, 1994, discloses the isolation and characterization of nucleic acid fragments encoding fatty acid desaturase enzymes, and their use to produce oil seeds with altered levels of unsaturated fatty acids.
World Patent Publication WO90/10380, published on Sep. 20, 1990, discloses a homogeneous assemblage of mature rapeseeds having an oleic acid content of at least 79 weight percent with respect to total fatty acids and an erucic acid content of not more than 2 weight percent. These seeds are alleged to yield a vegetable oil having high heat stability; this vegetable oil may be used as a frying oil.
European Patent Publication EP 323,753, published on Jul. 12, 1989, discloses mature rapeseeds having, by weight, a total fatty acid content a high oleic acid content of at least 79% and not more than 2% erucic acid. The oil derived from these seeds is said to have increased heat stability.
"Clear Valley.RTM. Canola Oil" Technical Bulletin No. SA 2069 (1995 Cargill Foods) discloses a natural canola salad oil with low levels of linolenic acid and an oxidative stability of at least 25 AOM hours.
"Van Den Bergh Design NH" Technical Bulletin No. FI93184 discloses a natural canola oil with a high level of oleic acid and an oxidative stability of at least 20 AOM hours.
"SVO Trisun 80" Technical Data Sheet discloses a natural sunflower oil with 80% oleic acid and an oxidative stability of at least 35 AOM hours. "SVO Trisun Extra" Technical Data Sheet discloses a natural sunflower oil with 85% oleic acid and an oxidative stability of at least 60 AOM hours. "SVO HS-Natural" Technical Data Sheet discloses a high oleic (80%) sunflower oil with added natural tocopherols (0.2%) and an oxidative stability of at least 60-70 AOM hours.
"Kraft Food Ingredients Soy.cndot.LL" Technical Data Sheet discloses a low linolenic soybean oil with an oxidative stability of 23-25 hours.
Warner, K. et al. ((1989) JAOCS 66(4): 558-564) disclose the flavor and oxidative stability of soybean, sunflower, and low erucic acid rapeseed oils. The oxidative stability of non-hydrogenated soybean oil, held at 100.degree. C. for 1, 2, or 3 days, is reported as 13.5, 15.0, 14.0 AOM hours, respectively. White, P. J. and Miller, L. A. ((1989) JAOCS 65(8): 1334-1338) disclose the oxidative stabilities of low linolenic acid, high stearic acid, and common soybean oils. Mounts, T. L et al. ((1989) JAOCS 65(4): 624-628) disclose the effect of altered fatty acid composition on soybean oil stability.
U.S. Pat. No. 4,627,192 discloses a sunflower seed having an oleic acid content of 80% or greater. U.S. Pat. No. 4,743,402 discloses a high oleic sunflower oil.
FR 2617675, published on Jan. 13, 1989, discloses groundnut seeds with an oleic acid content of 74-84% and linoleic acid content about 2-8%. The low linoleic acid content is reported to ensure high storage stability.
World Patent Publication WO91/11906, published on Aug. 22, 1991, discloses safflower seeds having an oleic or linoleic acid content of at least 80%.
Oxidative stability is also an important characteristic for industrial oil applications. This problem is particularly acute for triglyceride oils which tend to deteriorate easily due to their high degree of unsaturation. The oxidation proceeds via a mechanism which is initiated by the formation of a free radical and occurs rather easily in triglyceride oils due to the high content of active methylene groups adjacent to the double bonds. The overall effect is a high susceptibility of the oil to oxidation, which is further complicated by contact of the oil with metals, such as iron and copper, present in the equipment or material to be lubricated. Metals act as catalysts in the oxidation process and accelerate degradation of the oil.
U.S. Pat. No. 5,580,482, issued to Chasan et al. on Dec. 3, 1996, discloses lubricant compositions stabilized against the effects of heat and oxygen.
U.S. Pat. No. 5,413,725, issued to Lal et al. on May 9, 1995, discloses pour point depressants for monounsaturated vegetable oils and for high monounsaturated vegetable oils/biodegradable base and fluid mixtures.
U.S. Pat. No. 5, 399,275, issued to Lange et al. on Mar. 21, 1995, discloses environmentally friendly viscosity index improving compositions.